Solid state membranes formed from ion-conducting materials continue to show promise for use in commercial processes for separating and recovering oxygen from oxygen-containing gaseous mixtures as well as for reacting oxygen formed therefrom with a feedstock comprising light hydrocarbons such as methane, natural gas, ethane or any available light hydrocarbon mixture. Representative solid-state membranes are those formed from mixed conducting multicomponent metallic oxides which are typically operated at high temperatures (e.g. 600.degree. C. or more) wherein the membranes conduct both oxygen ions and electrons. When a difference in oxygen partial pressure exists on opposite sides of the multicomponent metallic oxide membrane and operating conditions are properly controlled, oxygen is transported in the form of oxygen ions from the high oxygen partial pressure side to the low oxygen partial pressure side of the membrane while an electron flux occurs in the opposite direction of oxygen ion migration in order to conserve charge, producing pure oxygen on the permeate side of the membrane.
Fluid separation devices which employ solid-state membranes must exhibit a variety of mechanical and physical properties in order to provide sufficient operating life and to perform according to desired operating specifications. Fluid separation devices should be capable of being subjected to elevated carbon dioxide and water partial pressures without observing any appreciable loss in oxygen transport properties such as in the case wherein the device is heated by direct firing of a fluid feedstream which produces carbon dioxide, water and sulfur dioxide. Unfortunately, typical multicomponent metallic oxides known in the art provide varying degrees of these essential attributes.
In typical processes which employ fluid separation devices, a pressure differential exists across the solid-state membrane. Numerous multicomponent metallic oxides exhibit excessive creep at elevated temperature which may cause the solid-state membrane to deform and ultimately crack under an applied pressure differential. Some multicomponent metallic oxides degrade in the presence of sulfur dioxide at elevated temperatures. Therefore, the ceramist must endeavor to optimize the composition of such materials to achieve acceptable levels of each of the identified mechanical and physical properties.
Tabata and coworkers (J. Mat. Sci., 22 (1987) 1882-1886) investigated the surface chemical states of the perovskite-type compound, strontium-doped lanthanum cobalt oxides (La.sub.1-x Sr.sub.x CoO.sub.3) utilizing X-ray photoelectron spectroscopy. Catalytic oxidations of both methane and carbon monoxide were also studied using various flow methods.
U.S. Pat. No. 5,261,932, assigned to Air Products and Chemicals, Inc., presents a process for recovering oxygen from an oxygen-containing gaseous mixture containing one or more components selected from water, carbon dioxide or a volatile hydrocarbon. The process utilizes ion transport membranes comprising a stoichiometric multicomponent metallic oxide containing strontium, calcium or magnesium. The process utilizes a temperature regime which overcomes problems associated with degradation of strontium-, calcium- and magnesium-containing multicomponent oxides caused by carbon dioxide. Preferred stoichiometric multicomponent metallic oxides are represented by the structure A.sub.x A'.sub.x A".sub.x B.sub.y B'.sub.y B".sub.y O.sub.3-z, where A,A',A" are chosen from the group comprising Groups 1, 2 and 3 and the F block lanthanides; and B,B',B" are chosen from the D block transition metals according to the Periodic Table of the Elements adopted by the IUPAC wherein 0.ltoreq.x.ltoreq.1, 0.ltoreq.x'.ltoreq.1, 0.ltoreq.x".ltoreq.1, 0&lt;y.ltoreq.1, 0.ltoreq.y'.ltoreq.1, 0.ltoreq.y".ltoreq.1, x+x'+x"=1, y+y'+y"=1 and z is a number which renders the compound charge neutral. Preferably, A, A' or A" of the enumerated structure is a Group 2 metal selected from the group consisting of calcium, strontium and magnesium.
U.S. Pat. No. 5,269,822, also assigned to Air Products and Chemicals, Inc., presents a process for recovering oxygen from an oxygen-containing gaseous mixture containing one or more components selected from water, carbon dioxide or a volatile hydrocarbon. The process utilizes ion transport membranes comprising a stoichiometric multicomponent metallic oxide containing barium and a temperature regime which overcomes problems associated with degradation of barium-containing multicomponent metallic oxides caused by carbon dioxide. Preferred stoichiometric multicomponent metallic oxides are represented by the structure A.sub.x Ba.sub.x B.sub.y B'.sub.y B".sub.y O.sub.3-z, where A is chosen from the group comprising Groups 1, 2 and 3 and the F block lanthanides with the proviso that A is not yttrium; and B,B',B" are chosen from the D block transition metals according to the Periodic Table of the Elements adopted by the IUPAC wherein 0.ltoreq.x.ltoreq.1, 0&lt;x'.ltoreq.1, 0&lt;y.ltoreq.1, 0.ltoreq.y'.ltoreq.1, 0.ltoreq.y".ltoreq.1, x+x'=1, y+y'+y"=1 and z is a number which renders the compound charge neutral.
U.S. Pat. Nos. 5,356,728 and 5,580,497 and European Patent Application WO 94/24,065 disclose cross-flow electrochemical reactor cells formed from multicomponent metallic oxides of the perovskite structure which demonstrate both electron conductivity and oxygen ion conductivity at elevated temperatures. Such cells are useful in carrying out partial oxidation reactions of organic compounds to form added-value products and separating oxygen from oxygen-containing gaseous mixtures. Suitable multicomponent metallic oxides are represented by EQU (Sr.sub.1-y M.sub.y).sub..alpha. (Fe.sub.1-x Co.sub.x).sub..alpha.+.beta. O.sub..delta.
where M is a metal selected from the group consisting of elements having atomic number in a range from 56 to 71, calcium, and yttrium, x is a number in a range from about 0.01 to about 0.95, y is a number in a range from about 0.01 to about 0.95, .alpha. is a number in a range from about 1 to about 4, .beta. is a number in a range upward from 0 to about 20, such that EQU 1.1.ltoreq.(.alpha.+.beta.)/.alpha..ltoreq.6,
and .delta. is a number which renders the compound charge neutral, and wherein the composition has a non-perovskite structure.
U.S. Pat. No. 5,306,411 discloses a solid multicomponent membrane for use in an electrochemical reactor characterized by (1) an intimate, gas-impervious, multi-phase mixture of an electronically-conductive phase and an oxygen ion-conductive phase wherein at least one of said phases is a mixed metal oxide having a perovskite structure represented by the formula EQU A.sub.s A'.sub.t B.sub.u B'.sub.v B".sub.w O.sub.x
wherein A represents a lanthanide, Y, or mixture thereof; A' represents an alkaline earth metal or mixture thereof; B represents Fe; B' represents Cr, Ti, or mixture thereof and B" represents Mn, Co, V, Ni or Cu, or a mixture thereof; and s, t, u, v, w, and x each represent a number such that:
s/t equals from about 0.01 to about 100; PA0 u equals from about 0.01 to about 1; PA0 v equals from about 0.01 to 1; PA0 w equals from zero to about 1; PA0 x equals a number that satisfies the valences of the A, A', B, B' and B" in the formula; and PA0 0.9&lt;(s+t)/(u+v+w)&lt;1.1; PA0 s/t equals from about 0.01 to about 100; PA0 u equals from about 0.01 to about 1; PA0 v equals from about 0.01 to 1; PA0 w equals from zero to about 1; PA0 x equals a number that satisfies the valences of the A, A', B, B' and B" in the formula; and PA0 0.9&lt;(s+t)/(u+v+w)&lt;1.1.
or (2) a mixed metal oxide material having a perovskite structure represented by the formula EQU A.sub.s A'.sub.t B.sub.u B'.sub.v B".sub.w O.sub.x
wherein A represents a lanthanide or Y, or a mixture thereof; A' represents an alkaline earth metal or a mixture thereof; B represents Fe, B' represents Cr or Ti, or a mixture thereof; and B" represents Mn, Co, V, Ni or Cu, or a mixture thereof and s, t, u, v, w, and x each represent a number such that:
F. Morin and coworkers (Solid State Ionics 96 (1997) 129-139) studied the phase stability of La.sub.0.5 Sr.sub.0.5 CoO.sub.3-.delta. (strontium-substituted lanthanum cobaltite). In preparing various ABO.sub.3 compounds with A=La and Sr and B=Co or Mn, the researchers found that strontium-substituted lanthanum cobaltite has much less tolerance for any variation in the A/B ratio than its manganite counterpart. This is specifically demonstrated for La.sub.0.5 Sr.sub.0.5 CoO.sub.3-.delta. where distinct phases readily appear on either sides of the A/B ratio for any small departure of this ratio from unity. These secondary phases are clearly evidenced by scanning electron microscopy and X-ray diffraction. The same observations have been extended to various degrees of strontium substitution between 0.4.ltoreq.x.ltoreq.0.6. The researchers also provide additional data in regard to the main phase stability as a function of the oxygen partial pressure at temperatures up to 1425.degree. C.
D. Waller and coworkers (Materials Letters 27 (1996) 225-228) discuss the structure of and reaction of A-site deficient perovskites. Lanthanum strontium cobalt iron oxides with the perovskite structure were synthesized using citrate and glycine complexation methods. Low temperature calcination of the precursor phases was stated to lead to the formation of cubic perovskites, which on high temperature calcination are stated to form rhombohedrally distorted perovskites. The cubic phase is identified as a perovskite La.sub.0.6 Sr.sub.0.4-x Co.sub.0.2 Fe.sub.0.8 O.sub.3-.delta., with a large degree of strontium deficiency (x=0.20 to 0.25).
Japanese Patent Application Kokai No. H8-130018 presents A-site deficient perovskites represented by the formula A.sub.1-.alpha. BO.sub.3-.delta.. Such materials are suitable for use as electrode material for solid electrolytes characterized by the fact that A in the formula consists of two structural elements A' and A", B consists of two structural elements B' and B", so that the general formula can be indicated by (A'.sub.1-x A".sub.x).sub.1-.alpha. (B'.sub.1-y B".sub.y)O.sub.3-.delta., and that A' in the formula consists of at least one selected from the group of La, Nd and Y; and A" consists of at least one selected from the group of Ba, Sr and Ca; B' is Co; and B" consists of at least one selected from the group of Mn, Fe, Ni and Cu, and the allowable ranges of .alpha., .delta., x and y are, 0&lt;.alpha.&lt;0.2; 0.ltoreq..delta..ltoreq.1; 0&lt;x&lt;1, and 0&lt;y&lt;1.
Those skilled in the art are searching for mixed conducting multicomponent metallic oxides of the perovskite-type for use in fluid separation devices which would tolerate being subjected to high carbon dioxide and water partial pressures and the presence of sulfur dioxide during operation without suffering an unacceptable loss in oxygen flux or exhibiting unexceptable creep.